auth_core: Add osmo_auth_3g_from_2g() to compute CK+IK from Kc

This function performs the C5+C4 conversion to derive UMTS key
material from a 2G-only subscriber.
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 5ed9499..5284842 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -231,6 +231,7 @@
 osmo_auth_alg_parse;
 osmo_auth_gen_vec;
 osmo_auth_gen_vec_auts;
+osmo_auth_3g_from_2g;
 osmo_auth_load;
 osmo_auth_register;
 osmo_auth_supported;